1. Technical Field
The present invention relates to an ion storage apparatus capable of being used as a mass analyzer, and in particular, to a linear ion trapping apparatus capable of being used as a linear ion trap mass analyzer, and an array structure thereof.
2. Related Art
The mass spectrometry method is one of important analysis methods in the current mainstream fields of chemistry and life science. As a main analysis apparatus of the mass spectrometry method, a mass spectrometry instrument has been developed from a desktop type to a portable type, and even to a handheld type in recent years. The development of these new mobile devices has new requirements for miniaturization of major components of the mass spectrometry instrument, especially, the mass analyzer which functions as the core of the mass spectrometry instrument. The main objective is to ensure the basic analysis performance of the mass analyzer while ensuring the miniaturization and simplification of the structure.
Meanwhile, peripheral components such as a vacuum cavity and an acquisition system also limit the development of a portable mass spectrometry method. Among many types of mass analyzers, the ion trap mass analyzer has features such as a simple structure and a small volume. Moreover, this type of mass analyzer has a lowest requirement on the working vacuum degree among various types of mass analyzers at present. Therefore, in the application of the portable mass spectrometry instrument, instruments that use an ion trap mass analyzer as a core component plays a major role.
Both the ion trap mass analyzer and quadrupole rod mass analyzer that serves as a mainstream mass spectrometry instrument analyze ions under test based on the trajectory stability of different ions in a quadrupole RF(RF) trapping electric field. According to the spatial structure characteristic of quadrupole trapping electric fields, quadrupole trapping electric fields can be classified into axis-rotation symmetry three-dimensional quadrupole trapping electric fields and axis-translation symmetry two-dimensional quadrupole trapping electric fields. For an ion trap mass analyzer, these two types of internal electric field structures correspond to two basic types, that is, a three-dimensional ion trap and a linear ion trap. The ion trap structure that appeared first is the three-dimensional ion trap. Because of the structural characteristic of the electric field, the structure of this type of ion trap is mainly formed by rotating bodies, and therefore, lathe machining may be adopted for the machining technique of this type of ion trap. In lathe machining, the implementation of the three-dimensional structure only requires determination through the displacement of a lathe cutter on a two-dimensional r-z plane that passes through rotation axis z, that is, the surface of an ideal three-dimensional ion trap is made to be a hyperboloid of revolution; the three-dimensional structure can also be implemented by means of numerically-controlled lathe machining, and the machining precision can easily reach about 1 micron, which meets the current basic level of precision machining technique in China. At present, portable mass spectrometry instruments that use three-dimensional ion traps as mass analyzers are available in China.
However, before being analyzed, ions gather at the structural center of the three-dimensional ion trap in a dot-like distribution. Due to a space charge effect caused by a coulomb repulsive force between ions, the number of ions that can be stored by the three-dimensional ion trap is relatively limited. In addition, during ion analysis, the gathering of a large quantity of ions in the space changes a trapping electric field in the trap, and especially the gathering of many ions at the central part of the trap causes a greater impact on the potential distribution at the part. The upper limit of the number of ions stored in the three-dimensional ion trap generally does not exceed 106 to 107. When the number of stored ions exceeds 5×104 or the number of ions having the same mass-to-charge ratio exceeds 5×103, the mass resolution capability of the three-dimensional ion trap decreases by a great degree, which significantly affects the dynamic range of the ion trap as a mass analyzing tool. Moreover, introduction efficiency of ions having different mass-to-charge ratios has an obvious relationship with their introduction RF phases, which also causes an obvious decrease in sensitivity when an external ion source is used. In addition, when an abundance spectrum of broken ions is used as a qualitative standard, the analysis structure thereof becomes unreliable due to the mass discrimination process.
In the mid 90s, John E P Syka et al. from the US company Finnigan proposed a two-dimensional linear ion trap structure to solve the foregoing problem. In the linear ion trap, ions are gathered near a central axis by a substantial two-dimensional quadrupole RF electric field. Therefore, with the same space charge density, the linear ion trap can accommodate much more ions. The two-dimensional linear ion trap can store ions more than those the three-dimensional ion trap can store by at least one order of magnitude, and can avoid an obvious impact from the space charge effect. Documents in recent years indicated that when a linear ion trap stores millions of ions, the mass spectrum resolution capability is still not affected. In the original patent file U.S. Pat. No. 5,420,425, Syka et al. pointed out that it requires at least two electrodes extending in the axial direction to implement such a structure. However, due to the need for constructing an ideal two-dimensional quadrupole field, a common linear ion trap has a “quadrupole rod” symmetric structure as shown in FIG. 1. Voltages output by a group of RF power sources 101 and 102 that are inverted to each other are applied on an electrode pair 12 and 14 and an electrode pair 11 and 13, respectively, so as to provide a radial trapping RF quadrupole electric field; axial motion of trapped ions is trapped by a group of end electrodes 15 and 16. Similar to the quadrupole rod mass analyzer, this ion trap needs to be driven by a pair of RF voltage sources 101 and 102 having opposite phases. Different from the quadrupole rod mass analyzer, to confine axial motion of ions, this ion trap needs to be provided with end electrode structures 15 and 16 at the front end and rear end of the axis, so as to restrict ion motions by means of voltages on the electrode structures. As regards machining characteristics, electrodes of a linear ion trap need to be machined by using a high-precision curved-surface grinding machine, and the machining is more difficult than that of a three-dimensional ion trap. In addition, assembly of electrodes 11, 12, 13, and 14 cannot be implemented by using a rotary insulator structure of a three-dimensional ion trap; to assemble the electrodes, special-shaped fit slot and key structures need to be provided on an internal cylindrical surface that supports an insulator, which makes the overall process more complex and exceeds the general precision machining level in China.
An important characteristic of the quadrupole trapping electric field is that the distribution of its space potential is a quadratic function related to the distance to the center of the field. Therefore, during vibration of ions in the electric field, the restoring force on ions satisfies the Hooke's law, that is, the vibration demonstrates characteristics of simple harmonic vibration. Generally, the last step of the analysis process of the linear ion trap is that ions sequentially resonate with an auxiliary excitation voltage based on mass-to-charge ratios thereof, so as to leave the ion trap via the slot-shaped slit provided on the electrode of the linear ion trap and be detected by an ion detection apparatus to form a mass spectrum. Due to the existence of the slot-shaped slit, some of the space potential near the slit is missing as compared with the space potential formed by a complete hyperboloid electrode structure, that is, the field intensity near the ejection slot decreases. Such a change of the space electric field can be expressed by a series expansion ΣAnRe(x+yi)n of the harmonic function of the space pseudo potential in the trap, where x is an ejection direction of ions, y is another direction orthogonal to the axis of the ion trap and the ejection direction, the term A2 is a component of a quadrupole field, and the term An is a component of a 2n-pole field. After an ejection slot structure 17 is added, ions in the ion ejection direction are affected by a negative RF high-order field generated by the loss of the RF electric field near the slot. The direct impact of the negative high-order field on ion motion is that, when the vibration amplitude of the ions increases, the resonance frequency of the ions has a red shift. As the mass scanning is generally performed from a low mass-to-charge ratio toward a high mass-to-charge ratio, the ion motion frequency has a blue shift along with the scanning process. The red shift process detunes the motion resonance of ions and therefore slows down the ejection process, causing a loss of the mass resolution.
To solve the foregoing problem, the inventor of the linear ion trap uses a so-called stretch structure, that is, the spacing between opposite electrodes in the ion ejection direction X is stretched outward symmetrically relative to the boundary of an ideal quadrupole field. This operation produces a positive-An high-order electric field in the ion ejection direction. In a normal mass scanning process, if the motion frequency of ions of any specific mass-to-charge ratio has a constant blue shift process, that is, the motion frequency moves toward a higher frequency, the introduced positive high-order field can produce the following advantages on the mass analysis process of the ion trap: First, when ions resonate at the center of the trap, the resonance frequency has a blue shift because the vibration amplitude increases at the beginning of the resonance. After that, at an appropriate scanning speed, this blue shift effect is synchronized with a natural blue shift process of the ion motion frequency, so that ions always resonate effectively during the ejection motion frequency shift process and the ejection is accelerated, and therefore, the mass resolution of the linear ion trap when being used as a mass analyzer is ultimately improved. Generally, to achieve this objective, the stretching ratio of the electrode structure is set to about 3% to 10% of the radius of the original hyperboloid quadrupole field, where the field radius refers to a saddle point of the substantial quadrupole electric field, or a distance from the center of the electric field to the boundary electrode. It should be pointed out that, the finally commercialized linear ion trap solution designed by Jae Schwartz et al. has an x-y plane symmetric structure, and the probabilities of the ion ejection process in direction x are consistent. Therefore, in their commercial instruments, a pair of detector groups disposed on two sides of the linear ion trap is used to obtain a mass spectrum, so as to achieve maximum ion detection efficiency.
J. Hager from Sciex later provided another axial ejection linear ion trap technology. In this technology, ions mass-selectively leave the linear ion trap along an axial end direction of a substantial quadrupole rod structure. Because ions do not need to leave in the axial direction, it is unnecessary to provide slots on the rod electrode. Therefore, influence of an adverse factor such as a negative high-order field on the field pattern and device performance is avoided. In this technology, when the RF fringing field at an end of the substantial quadrupole rod structure and a DC (Direct Current) electrode 15 at the tail end of the quadrupole rod structure form a suppression electric field to eject and block ions, a combined effect changes from blocking to ejection as the radial coordinate of the ion increases, so as to achieve a mass selection process for ion ejection at the axial end. The advantage of this technology is that, this ion trap has no boundary electric field deficiency caused by the ejection slot, and therefore, can also be used as a common quadrupole mass filter; the disadvantage lies in that, ions are ejected at the axial fringing field only when moving to the tail end of the trap, and therefore, under the condition of a high scanning speed, ions in the trap can be ejected only when they are at the tail end of the trap; otherwise, ions are lost on the rod electrode, which causes the maximum scanning speed and ion detection efficiency thereof to be lower than those of the radial ejection process previously proposed by Schwartz et al.
Two existing basic linear ion trap working manners are described above. To simplify and improve the trap electrode structure, Professor Ouyang zheng and Professor R G. Cooks, et al. from Purdue University suggested, in the U.S. Pat. No. 6,838,666 pre-applied in 2003, replacing, in the substantial quadrupole rod structure of the original linear ion trap, the hyperboloid or rod electrode structure in the original commercial instrument with a planar electrode, so as to form a rectangular linear ion trap mass analyzer. The machining of the planar cylindrical electrode structure is relatively simple, and therefore, the mass analyzer is easier to implement under the same machining precision. The disadvantage of this structure lies in that, due to the ion trap cross section structure formed by the rectangular planar electrode, a lot of high-order field effect is introduced in the trap. In addition, the trap still uses the x-y plane symmetric structure, and uses a non-integer-divider dipole excitation auxiliary RF located at a non-linear resonance band of an octupole field. Therefore, according to the principle, ion ejection probabilities on ejection direction X of the mass analyzer are still almost the same, and to obtain the highest ion detection efficiency, a pair of detector groups disposed on two sides of the linear ion trap is still used to obtain a mass spectrum.
Further, as regards the electric field deficiency caused by the rectangular planar electrode structure as compared with the hyperboloid structure, in 2004, Ding Chuanfan et al. from Fudan University suggested, in Chinese Patent 200410024946.8, producing an ion trap by using a common printed circuit board, and adjusting the field pattern in the trap by applying RF voltages of different amplitudes on electrodes on the surface of the printed circuit board. Compared with the design of the rectangular ion trap, the printed circuit board ion trap has less high-order field component, and the quadrupole field component can reach 98%; under the same RF amplitude, the electric field intensity at the center of the trap is stronger than that of the rectangular ion trap, and therefore the trapped ion cloud has a better collision focus effect. The ion trap in this design is formed by four completely enclosed PCB flat electrodes and two thin electrode end caps having support legs.
To further simplify this design, Ding chuanfan et al. further proposed an ion storage and analysis apparatus array in Chinese Patent Application 200610001017.4 and US Patent Application 2009/0294655 A1, which includes two or more rows of electrode arrays disposed parallel to each other, where strip-shaped electrodes in the electrode array are parallel to each other. High-frequency voltages of different phases are applied on adjacent electrode strips, so that a high-frequency electric field is generated in a space between two electrode arrays, thereby forming multiple parallel linear ion trapping areas in this space. According to the result published by the inventors on the US journal Analytical Chemistry, the linear ion trap array still has the same mass resolution effect as that of a simply piled up rectangular ion trap array structure while saving surrounding electrodes in a direction (direction y) orthogonal to the ejection direction for each storage unit, and the structure is more compact. Moreover, as the electrode units in the direction orthogonal to the ejection direction are omitted, the possible mechanical structure error caused by these electrode units is also avoided.
Compared with a multi-layer concentric ion trap array structure previously proposed by others, for example, the cylindrical ion trap array proposed by the R G. Cooks Research Group from Purdue University in U.S. Pat. No. 6,762,406 and the later cylindrical ion trap array mass analyzer chip produced by Ramsy et al. based on the micro-electromechanical systems (MEMS) technology, the device proposed by Ding chuanfan et al. has unique features, such as a large ion storage capacity, of a linear ion storage apparatus. It should be pointed out that, the area required by the detector of the ion trap array is similar to that of the previous ion trap array design, which is approximately the area occupied by the body of the ion trap array. This is adverse to the multi-channel synchronous detection of the ion trap array; that is, the analysis process in which multiple ion traps store ions separately, and then eject ions selectively based on a same mass axis; this is because a larger detector area means a greater collector capacitance during coulomb detection. Therefore, relative to the detector design for detecting a single ion trap, a transient voltage response caused by the ion current declines. Of course, this problem can be solved by setting multiple separate detector units. However, multiple detector units require cooperation of multiple sets of post-amplification and analog-to-digital circuits, which increases complexity during an actual application.
In the Chinese Patent Application 200910054963.9, Pan Xinyuan et al. from Fudan University further simplify the electrode structure of the above ion trap having a PCB electrode parallel array structure designed by Ding chuanfan et al. In the structure designed by Pan Xinyuan et al., two parallel PCB circuit boards are used, and each PCB circuit board has a RF planar electrode, where the two RF planar electrodes correspond to each other so as to form a substantial quadrupole trapping electric field in the space of the pair of RF planar electrodes. To improve the field pattern of the structure, two end cap electrodes on the same plane are designed on two sides of the plane of each RF electrode. The combined function of the two end cap electrodes on the same side substitutes the function of the electrode in direction Y that is orthogonal to the ejection direction of the original “substantial quadrupole rod” structure. However, the mass resolution performance of such a design result is unsatisfactory; it can be seen from the spectrogram of an electron bombardment ionization source about perfluorotributylamine, this structure only obtains unit mass resolution in a mass range of less than 200 Thomson.
The above designs of the linear ion trapping apparatus used as the mass analyzer are all established based on X-Y bisymmetric geometric structures, and in these structures, the mass analyzer has the same ion ejection probability at two sides along the ejection direction x. To improve the ejection characteristic of the substantial quadrupole rod linear ion trapping apparatus, Franzen et al. proposed a substantial quadrupole rod ion trap in the U.S. Pat. No. 6,831,275, and in the ion ejection direction, asymmetric high-order multi-pole field component additions such as a hexapole field and a decapole field are obtained by modifying the structure or voltage of the original quadrupole field, and by using the characteristic that the non-linear resonance thereof is asymmetric at the positive part and negative part in direction x, the ion mass selectivity and ejection efficiency at the axial end are improved. These characteristics are mentioned again by DJ Douglas et al. in the U.S. Pat. No. 7,141,789, and it is pointed out that, with 1% to 10% of the hexapole field addition, ions can be selectively lost on the rod electrode by means of asymmetric non-linear resonance in direction x, so as to improve the ion selectivity of the axial end ejection. However, these in the prior art all relate to the substantial quadrupole rod structure only, and setting of the orientation characteristic of ion ejection during a radial mass-selective ejection process of ions is not discussed.
Gregory J. Wells from Varian provides another method in the U.S. Pat. No. 7,034,293, in which the ion trapping center of the trap is deviated from its geometric center by changing the DC voltage configuration applied on the substantial quadrupole rod structure, so as to mitigate the asymmetry of the radial ejection of the ion trap. However, according to the fundamental quadrupole trapping apparatus theory, after a DC bias is applied, a certain quadrupole DC electric field is applied on the cross section of the ion trap, which causes a high mass loss phenomenon to result in mass discrimination when ions are introduced, and affects the full-mass scanning performance.
Li Ding et al. proposed a field adjustment electrode disposed outside an ion trapping apparatus in Chinese Patent Application 200910253112.7, so as to improve the ejection direction selectivity of this type of apparatuses. This method adjusts the ejection characteristic of ions by using a DC bias. In this solution, the electrode is located on an external side of the trapping apparatus, and therefore, the voltage change of the electrode has a smaller effect on the center of the trapping apparatus. Compared with the solution in the patent of Varian, this solution significantly mitigates the problems such as mass discrimination. However, in the embodiment of this patent application, only a linear ion trap system having a common substantial quadrupole rod structure is described.